The present invention relates to a pressure tolerant transducer for transforming energy from one form to another, for example for transforming electrical energy into acoustic energy or vice versa.
Transducers, for example acoustic transducers, are well known in the prior art.
In a published international patent application no. WO 98/53924, there is disclosed a flexural plate sound transducer comprising a housing having an open central volume, a flexural plate attached around an inner surface of the housing and extending across the central volume, at least one piezoelectric element attached to a surface of the flexural plate. A mechanical hinge is formed near an outer periphery of the flexural plate and extends around the flexural plate. The mechanical hinge is formed such as to cause the flexural plate to move in a substantially piston-like manner when the piezoelectric element is energised. In the published application, the flexural plate and its associated at least one piezoelectric element are of uniform thickness except in the region where the hinge is formed.
In a published European application EP 0 264 557 A2, there is described a piezo-ceramic sound transducer comprising a metal membrane onto which is bonded in a central region thereof a piezo-ceramic slice. The membrane includes a support ring at a peripheral region thereof, and also a concentric ring-form compliant grove in the membrane between the slice and the support ring. The membrane is of uniform thickness even in the grove.
In a U.S. Pat. No. 5,724,315, there is described an omni-directional ultrasonic microprobe hydrophone. The hydrophone comprises two sensing elements where each element is composed of lead zirconate titanate and includes a plurality of columnar voids. In the hydrophone, the elements are bonded to an associated substrate material in the form of a backing plate. The voids are located in the hydrophone between the backing plate and the elements, the voids forming compressible cavities.
The inventors have appreciated that transducer structures known in the art often experience difficulties coping with relatively elevated environmental pressures applied thereto. Thus, the inventors have developed an alternative transducer exhibiting enhanced resilience to elevated environmental pressure, for example as experienced at a depth in the order of 200 m in aquatic environments.
The present invention seeks to provide a transducer incorporating a plate structure which is so constructed that it provides a workably low resonant frequency and which is capable of operating under extreme conditions, for example at large depths underwater in the order of 200 m.
According to a first aspect of the present invention, there is provided a transducer comprising a layer of active material, a backing plate having first and second major surfaces, to the first surface of which is affixed a layer of active material, and a region adjacent to the second major surface into which region the backing plate can be deflected, the region being substantially isolated from any external pressure incident on the layer of active material, wherein the backing plate and/or layer of active material is of a non-uniform thickness.
The invention provides the advantage that the transducer is capable of being used in applications where the transducer is exposed to relatively high external pressures.
One specific usage is where hydrostatic pressure is encountered when the transducer is used underwater. Thus, a transducer of the invention which acts in use to transform electrical energy into acoustic energy may be utilised as a xe2x80x98projectorxe2x80x99 in a sonar system where the acoustic energy is broadcast into water. Alternatively, or in addition to, the transducer of the invention may be utilised as a hydrophone in a sonar system where it acts to transform acoustic energy into electrical energy.
Other applications envisaged for the transducer include diver-to-diver, ship-to-diver and ship-to-ship communications systems, and ships in these contexts should be understood to include xe2x80x98submarinesxe2x80x99.
The transducer with which the present invention is concerned is of the type which includes a plate structure comprising a backing plate to at least one side, namely a major surface, of which an active material is applied. In the context of the present invention, an active material is defined as:
(a) a polarised or unpolarised material, such as lead-zirconate titanate, lead titanate, barium titanate, lead metaniobate, lead magnesium niobate/lead titanate (typically either ceramic or single crystalline) or nickel;
(b) a piezo-electric material, such as crystalline quartz; or
(c) a magnetostrictive material, such as Terfenol-D.
When the transducer is in use, the active material is deformed by the application of energy in one form and converts that energy into a different form. Thus, in one type of transducer for example, an alternating potential is applied to the two major surfaces of the active material plate via metal electrodes. Such excitation produces an alternating electrical field across the thickness of the active material plate. In response to this field, the plate attempts to expand or contract in the direction of its plane, that is radially in the case of a disc-shaped plate. The backing plate, to which the active material plate is bonded, constrains most of the said strain at or near the bond line. The side of the active material plate remote from the bond line, however, remains reasonably free to expand and contract. The composite plate therefore undergoes periodic flexure. In the case of an underwater projector, this movement is communicated to the surrounding water, and the energy is propagated away as sound.
In transducers where an active material plate is bonded to each side of a backing plate, the electrical field will be applied to each active material plate in such a phase relation so that the active plates flex in opposite directions, thereby reinforcing each other""s action.
Because of its flexural action, such a transducer is sometimes referred to as a xe2x80x98benderxe2x80x99.
The invention provides an improved plate structure which overcomes problems associated with conventional prior art plate structures.
In this regard FIG. 1A shows a section through one face of a prior art plate structure, and FIG. 1B is a plan view of the plate structure of FIG. 1A.
The plate structure 1 comprises a backing plate 10 which is flat and of uniform thickness. The backing plate 10 is generally symmetrical; the plate 10 is shown as circular but other shapes are possible. Attached to at least one side of the backing plate 10 is a layer 11 of the active material, for example a polarised electrostrictive material. The layer 11 as shown itself takes the form of a circular plate which is flat and of uniform thickness and which is attached to the backing plate 12 by suitable attachment means 13.
As shown, the layer 11 is of such a size that there is an annular area 12 of the backing plate 10 adjacent the outer circumference thereof which is free of active material, although such an area does not necessarily have to be provided.
It is further known to have layers, for example in plate form, of the active material on both sides of the backing plate; such a configuration is described in a published international application WO 98/53924.
The backing plate 10 may be supported on a support structure which can take various forms as shown below.
Further and as illustrated in FIG. 2 it is known to utilise two plate structures of a type illustrated in FIG. 1 in a transducer. In FIG. 2 the two structures 1 are separated by an annular support element 20 which is affixed at or near the outer circumference of the backing plate(s) 10 to support and separate the two plate structures. The space between the plate structures can be filled with a gas (for example air) or liquid.
Such a prior art structure will, when used underwater, experience hydrostatic pressure from the water, which pressure will increase with depth; such pressure is liable to cause deleterious effects on the structure. Depending on the precise make-up of the structure, there will be a limit to the depth at which the transducer can be used before one or both of the following deleterious consequences ensue:
(i) the backing plate and/or the active plate will physically collapse;
(ii) in the case of polarised active material, the material will suffer depolarisation.
A requirement of a sonar device is that it shall be capable of delivering useful quantities of acoustic power, with a reasonable level of efficiency. In a sonar device incorporating a composite plate structure, the value of the resonance frequency will be determined by the stiffness and masses of the components of the plate structure according to the equation:       v    0    =      1          2      ⁢      π      ⁢                        (          MC          )                    
where v0 is the fundamental resonance frequency;
M is the collective effective mass of the relevant components of the structure;
C is the collective effective compliance of the relevant components of the structure.
It is to be noted that compliance is the inverse of stiffness.
The achievement of a low fundamental resonance frequency requires, therefore, large mass and/or large compliance. The compliance of the device is approximately proportional to {1/(thickness)3} of the active material plate and the backing plate; therefore thickening a plate in order to allow the device to operate at greater maximum depth will reduce its effective compliance substantially more than it will increase its effective mass, hence raising the fundamental resonance frequency of the device. There is, therefore, a conflict between the requirements to operate the transducer at a considerable depth and at low fundamental resonance frequency.
The present invention seeks to provide a transducer incorporating a plate structure which is so constructed that it resolves this conflict and which is capable of operating under extreme conditions, for example at large depths underwater in the order of 200 m.
In the transducer according to the first aspect of the invention, it is preferable that the backing plate is of non-uniform thickness.
Preferably the backing plate is supported around its periphery on a support member.
The backing plate may be thicker at a central region thereof than at an edge region thereof.
It is also envisaged that the backing plate may be formed with an outer lip portion of increased thickness relative to an inner region, and in such an arrangement the lip may be bonded to the support structure.
The active material may be encapsulated in a layer of a polymer material.
Also the transducer may have a recess adapted to receive a flexible elongate tensile member, for example a cable; such a recess may be formed in the layer of polymer material.
The support member may support two backing plates and associated active layers, the second major surfaces of the backing plates and the support structure defining a common region substantially isolated from any external pressure incident on the layers of active material.
It is envisaged that a hydrophone and/or projector may comprise a plurality of transducers as delineated above wherein two said transducers are linked by a cable, and wherein prior to deployment the cable is stored in a recess about the active layer with adjacent transducers being arranged together in close proximity such as to provide a housing for the cable prior to deployment.
According to a further aspect, the invention provides a transducer for converting one form of energy into another form of energy comprising a plate structure comprising a backing plate to at least one side of which is affixed an active material which plate structure incorporates a recess adapted to receive a flexible elongate tensile member.
The transducer of the invention may be operable in use to convert electrical energy into acoustic energy and/or may be operable in use to convert acoustic energy into electrical energy.
The transducer of the invention may be used underwater and may be included in a sonar system.